ocr: a The Machinery ofMovement Quit Muscle Some mollusk muscles, called catch mus- cles, can maintain tension for long periods with very little energ gy expenditure. Muissels and other bivalve mollusks that live on shores may have to keep themselves tight shut for several hours each day, while uncov- ered by the tide. If they opened, they would lose water. Their shells spring open whenever the adductor muscles (which close them) relax*so these muscles must remain active to keep the shell shut. A part of the adductor muscle is catch muscle, capable of going into a state in which it maintains tensio ...
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